Wireless communication technology is applied to the field of physical distribution management as well as that of information and communication. An IC tag for wireless communication (hereinafter simply referred to as “IC tag”) is well known as a product playing a role in an RFID (Radio Frequency Identification) technology. Since the IC tag is used in wide-ranging applications including physical distribution management and low-cost information storage medium, wireless communication equipment is used in various environments.
The IC tag includes a chip for storing data such as an identification number and an antenna for transmitting and receiving radio wave, which can be highly advantageously implemented in a thin and lightweight form. The IC tag may also be referred to as RFID tag or RF tag.
In order to make full use of the advantage, it would be desirable that the tag can be applied anywhere without limitation and is configured to be in communication no matter where and how the tag is applied.
However, the IC tag is designed to be used in free space. When radio wave in a very high frequency band, an ultra-high frequency band or a microwave band is used, a general-purpose tag performs transmission and reception in radio wave communication using what is called a dipole antenna. When a metal or the like exists near the antenna, the communication properties of the antenna is degraded, shortening the possible communication distance.
When a conductive material such as a metal exists near the antenna, a current flowing in the antenna induces a current flowing in the opposite direction in the metal, the induced current largely decreasing the input impedance of the antenna. This results in impedance mismatching with an IC chip designed for free space, shortening the possible communication distance. Also, when another current having the same amount exists near the current almost in parallel and the opposite direction, the direction of magnetic field generated around the current is opposite to that generated around the another current, the magnetic fields canceling each other, which prevents radio wave from being transmitted in long distance, i.e., shortening the possible communication distance.
Typically, a dipole antenna, a monopole antenna and a loop antenna are designed to be matched with the chip impedance in free, space when a resonance current flows in the IC chip that is generated in the antenna in response to receiving radio wave at a particular frequency.
FIG. 12 is a cross-sectional view showing a magnetic field generated near a wireless IC tag 20 that is located close to a conductive member.
When a communication disturbing member 112 exists near an antenna element 111, a current I11 flows to one end 111a from the other end 111b of the antenna element 111, and a current I12 flows from one portion 112a to the other portion 112b of the communication disturbing member 112, resulting in the currents flowing in the directions opposite to each other in the antenna element 111 and the communication disturbing member 112.
Since an alternating current is applied by an IC 117, a state that the currents flowing in the directions shown and a state of the currents flowing in the directions opposite to the shown directions occur alternately. FIG. 12 shows the directions of the currents generated at a given instant. When the frequency increases, a state occurs that is equivalent to that in which a current I0 flows between the one end 111a of the antenna element 111 and the one portion 112a of the communication disturbing member 112 and between the other end 111b of the antenna element 111 and the other portion 112b of the communication disturbing member 112. This state can be considered as that in which the one end 111a of the antenna element 111 and the one portion 112a of the communication disturbing member 112 are short-circuited in a high frequency manner, and also, the other end 111b of the antenna element 111 and the other portion 112b of the communication disturbing member 112 are short-circuited with respect to high frequency. When these short circuits with respect to high frequency occur, the antenna element 111 and the communication disturbing member 112 form a closed circuit, increasing the amount of the current in comparison with the case without the communication disturbing member 112 in the vicinity of the antenna element. In other words, the impedance decreases in comparison with the case without the communication disturbing member 112 near the antenna element 111.
Thus, when a conductive material exists near an antenna or chip, a current is induced on the surface of the conductive material in the direction opposite to that in which a current flows in the antenna, and furthermore, a high electric-field portion of the antenna and a low electric-field portion of the opposite surface of the conductive material, and a low electric-field portion of the antenna and a high electric-field portion of the opposite surface of the conductive material become connected with respect to high frequency, which generates a loop electric circuit of the antenna and the conductive material. The generation of the electric circuit significantly decrease the impedance, which causes a mismatch with the chip impedance, disabling information signal communication. This shortens the possible communication distance.
In addition to the metal, paper, glass, resin, liquid and the like may be a material that degrades the communication properties of the IC tag.
For these materials, the dielectric and magnetic properties of them cause the resonance frequency of the antenna to be changed, the difference between the frequency of radio wave used by the other communication party and the changed resonance frequency of the antenna shortens the possible communication distance.
Patent Literature 1 discloses an RFID tag including a power supply pattern with an LSI chip thereon; a patch antenna that operates as a tag antenna and a high-frequency coupler for coupling the power supply pattern and the patch antenna with respect to high frequency.